1. Field of the Invention
This invention relates to the inhibition of formation of scale from water containing scale forming minerals and more particularly relates to a method for inhibiting such scale by treating surfaces which are in contact with such water.
2. History of the Prior Art
The formation of scale from water containing minerals has been a historical problem and has especially been a problem when the water passes over surfaces having temperatures different than the water. Scale has especially been a problem when such water contacts heat exchangers which are either hotter or colder than the water passing through the exchanger. In such cases, when the heat exchanger surface is at a temperature which causes the water at the surface to be supersaturated with minerals, scale deposition is very rapid and cleaning or replacement of the heat exchangers is often necessary since the scale greatly restricts and sometimes entirely stops the flow of water through the exchanger. Furthermore, even when only a only thin layer of scale is deposited, the efficiency of the heat exchanger is greatly reduced since heat must travel through the scale as well as through the wall of the heat exchanger in order to accomplish the heat transfer function.
The formation of calcium sulfate scale in the exchangers, such as encountered in geothermal applications, evaporative desalination, cooling towers and petroleum engineering is an especially serious problem. Such scales are many times in the form of hard crystalline deposits and have been difficult to inhibit. The problem with calcium sulfate scale formation is made worse by a decreasing solubility of calcium sulfate, with increasing temperature.
Numerous attempts have been made to inhibit the formation of mineral scales on heat exchanger surfaces. These attempts have been more or less successful. Scale inhibiting chemicals have, for example, been dissolved in the water which contacts the heat exchanger. Examples of such chemicals are organic polyphosphonates and polyacrylates. The addition of such chemicals has serious disadvantages since such inhibition methods are not entirely successful and the heat exchangers must, nevertheless, continue to be cleaned or replaced more frequently than desirable. Furthermore, continuously supplying scale inhibiting chemicals to the water is wasteful of these chemicals, costly and requires monitoring to be certain that concentrations of scale inhibiting chemicals is sufficiently high.
There also have been attempts made to prevent scale upon heat exchanger surfaces by coating the surfaces with a substance having low surface energy such as polytetrafluoroethylene or silicones such as polydimethylsiloxane. Unfortunately, such polymeric materials form films which are sufficiently thick to reduce the efficiency of the heat exchangers by causing an insulation effect at the exchanger surface.
Other attempts at coating such heating exchanger surfaces have been made utilizing various kinds of oils which unfortunately rapidly wear from the surface thus leaving the exposed exchanger surface susceptible to scale deposition.